Eyewear lenses are optical parts for use with the human eye. This may include non-prescription eyewear, such as store-bought sunglasses; safety eyewear, such as spectacles or goggles that protect the eyes; and prescription eyewear prepared with corrective lenses that improve an individual's ability to focus light. Eyewear lenses preferably have an optimal combination of optical and physical properties to suit the user's needs. For instance, someone working with a high speed drill may be very concerned that their eyewear lens is impact resistant, but may not need (or want) tinting that would reduce light transmission through the lens. In contrast, the crew on a professional racing yacht may demand that their eyewear lenses reduce glare and improve their distance vision, but may be less concerned about other attributes. Thus, a broad range of properties may be considered important for various eyewear lenses.
Among the most important optical properties for eyewear lenses are refractive index and chromatic aberration. Refractive index characterizes how much a lens material bends or focuses light, and thus how thin a lens is needed to achieve a given optical power. Therefore, one would typically assume higher index is always better. However, refractive index is not constant with wavelength, and typically increases with decreasing wavelength. Hence, blue light will focus at a different physical distance than red light. This is called longitudinal or axial chromatic aberration. While this generates the rainbow effect we enjoy with a prism, it is problematic in eyewear lenses. In addition, for prescription eyewear that magnifies or reduces an image, one may also encounter chromatic aberration when one looks off-axis toward the edges of the lens. A prescription eyewear lens has different curvatures on the front and back surfaces of the lens, and the difference between these curvatures creates the lens' corrective power. As a result of the different curvatures, the center of the lens will be a different thickness than the edge. When the user looks off-axis, the effect is similar to looking through a tilted and wedged optical element with power. The wedge and tilt components act similar to a dispersive prism, again giving rise to chromatic aberrations. For eyewear lenses, both these types of aberration mean that the focused image will not be as clear or as sharply defined as it would be if all wavelengths of light focused to the same location. This chromatic aberration or dispersion is often perceived as blurring and/or color haloes around a viewed object.
A measure of the degree of chromatic aberration for a material is its Abbe number, Vx, expressed by the following relationship of refractive indices (nx) across the visible light region:
      ν    x    =            (                        n          green                -        1            )              (                        n          blue                -                  n          red                    )      For easier and more accurate comparison of the dispersion of different lens materials, the refractive indices used in the equation should be measured at the same wavelengths for each material. In the USA, the convention has been established to use the refractive indices measured at the following specific wavelengths: ngreen=587.56 nm, nblue=486.1 nm, and nred=656.3 nm, and the resultant Abbe number is referred to as Vd. Larger Abbe numbers correspond to less color spread, while smaller numbers indicate more color dispersion.
As one would expect from common experience with prisms, the dispersion effect is more noticeable for higher powered lenses, because edge sections of these lenses have more prism-like structure. Thus, patients with higher powered prescriptions (thicker lenses) may be more susceptible to the blur associated with chromatic aberration. Additionally, some persons may be particularly sensitive to color differences, and may find such dispersion more noticeable. Further, the dispersion is typically larger for higher index materials than lower index materials. Hence, when materials with a higher refractive index are used because dispensers want to create thinner lenses, they may inadvertently subject the wearer to more color and/or blur. Again, this illustrates there may be tradeoffs between lens materials' properties and factors considered important for various wearers.
It has always been a challenge to explain and illustrate these optical concepts effectively to those evaluating eyewear options. While it is easy to show people a rainbow display, the universal response is enjoyment, not concern with how that effect might blur one's vision. One alternative is to approximate what the eye may see via ray tracing and optical modeling with selected variables, and present simulated displays or values based on the theoretical calculations. U.S. Pat. No. 5,677,750 discloses methods that theoretically calculate what the eye views through a lens, assuming certain mathematical equations of the eye's theoretical response, input data or assumed values for the lens, and possible lighting. It also discloses an apparatus (such as a computer display) designed to create a simulation of the retinal image based on these calculations. U.S. Pat. No. 6,604,826 discusses methods and an apparatus to measure or input data from an eyewear lens, combine it with data on the eye's theoretical response, and generate or display theoretical regions that should correspond to comfortable vision based on calculations of specific values of visual acuity. Such techniques may create simulations or computer models that approximate a view through a lens, or may theoretically calculate and display areas that have been defined as clear vision, and then superimpose this model on a framed lens outline. However, people may not understand or trust computer simulations, or may perceive them as interesting games rather than real experiences.
Therefore, it is apparent that a need exists to directly demonstrate how a lens' dispersion may affect one's vision through the lens, by showing images of objects as viewed through the lenses. Ways to simultaneously compare different lenses are also advantageous because our human vision is much better at comparing differences when objects are viewed together, while our visual memory is less discerning. The present invention fulfills these needs and provides for further advantages.